The invention relates to an architecture for a hydraulic steering control system, intended in particular for fitting to an aircraft.
Aircraft generally include nosewheel landing gear having one or more wheels that are steerable in order to enable the aircraft to be taxied. For aircraft of large size, one or more steerable bogies are sometimes provided on the main landing gear, in addition to the steering device for the nose landing gear.
The steerable portions of landing gear are generally actuated by one or more actuators fed by the pressure-generator device of the aircraft via a hydraulic steering block situated close to the actuators, as a general rule directly on the landing gear. In conventional manner, the hydraulic steering block comprises a directional control valve, generally of the proportional type, serving to deliver fluid to the actuator(s) so as to control the steering of the steerable portion of the landing gear in response to orders from the pilot.
Steering is generally not considered as being a function that is critical from the point of view of aircraft safety. Loss of steering does not lead to catastrophic consequences, and the steering function can be compensated by differential braking, optionally associated with differential thrust from the engines. If necessary, the aircraft can be towed.
It is therefore common practice for the hydraulic steering block to be fed solely by the main hydraulic circuit of the aircraft, the hydraulic block being arranged to allow the steerable portion of the landing gear to turn freely when the aircraft is stationary or in the event of the pressure-generator device not operating.
Nevertheless, the loss of the steering function can interfere severely with aircraft operation. Controlling an aircraft that is taxiing by differential braking does not enable it to make sharp turns, and for aircraft of large size that is not necessarily compatible with the width available on taxiways. Furthermore, making sharp turns by blocking the wheels of the main landing gear on one side of the aircraft stresses said landing gear strongly in twisting which reduces its lifetime. In addition, requiring the use of a tractor to tow the aircraft can waste a great deal of time, and that can disturb the running of an airport in unacceptable manner.
In a conventional technique, the reliability of the steering function can be increased by duplicating the main feed circuit by means of an emergency feed circuit.
However, that solution when applied to the present situation presents numerous drawbacks. On large airliners, the hydraulic steering block of the nose landing gear is remote from the pressure-generator device of the aircraft by a distance of several tens of meters, and duplicating the pipework would give rise to harmful extra weight. Furthermore, segregation requirements make it essential for the main and emergency circuits to follow different paths through the structure of the aircraft, thereby complicating aircraft design.
In addition, the breakdown can come from the hydraulic block itself, and in particular from the directional-control valve. Duplicating the feed circuit in the conventional technique does not enable that breakdown to be remedied.
The state of the art is also illustrated by the following documents: WO-A-02/12052; WO-A-01/19664; GB-A-1 394 808; U.S. Pat. No. 2,874,793; DE-A-100 37 829; and DE-A-100 40 870.
Document WO-A-02/12052 describes a steering system architecture in which a steering actuator is fed in a normal mode of operation by a main pump via a directional-control valve, and under fault conditions by a reversible pump. No genuine alternate mode of operation is provided. For example, a breakdown can arise in which both pumps are delivering simultaneously into the actuator. In addition, if the main pump breaks down, there is no provision against the directional-control valve breaking down.
Document WO-A-01/19964 describes another architecture in which the normal and alternate modes of operation are provided by two reversible pumps. Thus, in the event of failure, those two pumps could likewise both deliver into the chambers of the actuator.
Document GB-A-1 394 808 describes another architecture in which the two modes of operation alternate only in the event of the main pump failing, without it being possible to cope with some other component of the hydraulic circuit failing.
Document U.S. Pat. No. 2,874,793 describes another architecture with manually operable valves to release the steerable portions so as to allow them to turn freely.
Document DE-A-100 37 820 describes yet another architecture having electrically-driven pumps connected to respective independent circuits. No means are provided to compensate for differential flows.
Document DE-A-100 40 870 describes an architecture that is complex, having two actuators in series, each possessing its own feed means.
The invention seeks to provide good reliability for the steering function of an aircraft but without suffering the drawbacks or limitations of the solutions described above.
The architecture for a hydraulic steering control system of the invention includes at least one steering control actuator having chambers, the hydraulic system comprising a directional-control valve connected to a pressure-generator device and an associated main supply, the hydraulic system further comprising a reversible electrically-driven pump unit having two ports, and the hydraulic system being fitted with a general selector arranged in a normal mode of operation to put the chambers of the actuator into communication with the directional-control valve, and in an alternate mode of operation to put the chambers of the actuator into communication with the ports of the pump unit, a compensation device making it possible in the alternate mode to compensate for the flow differential between the flow taken in by the pump unit from one of the chambers of the actuator and the flow delivered by the pump unit to the other chamber of the actuator.
Thus, in the event of the pressure-generator device of the aircraft breaking down, or in the event of the directional-control valve failing, the electrically-driven pump unit takes over to allow the aircraft to be steered.
By means of the hydraulic system of the invention, it is possible to overcome not only a breakdown of the generator device, but also a failure of the directional-control valve, without it being necessary to duplicate the main feed circuit.
Advantageously, the general selector is further arranged, in a passive, towing mode, to put the chambers of the actuator into communication with one another, the compensation device then compensating for any possible flow differential between the chambers of the actuator in the event of a maneuver being imposed on the actuator.
Passive mode allows the aircraft to be towed, the steerable portion of the landing gear then being free to turn without the actuator opposing such turning.
The compensation device is used both to compensate for flow rate differences between the ports of the electrically-driven pump unit when it is in operation, and for flow rate differences between the chambers of the actuators in passive mode.
This disposition makes it possible to avoid using an accumulator dedicated to the second above-mentioned compensation, as has been the practice in the prior art.
In a particular embodiment, the compensation device comprises a pressurized tank connected to each of the ports of the pump unit via an associated check valve making it possible during operation in alternate mode for hydraulic fluid to be transferred from the pressurized tank to one of the ports of the pump unit, and vice versa, each check valve being capable of being placed in a permanently open position by a respective pressure signal taken from the opposite port of the pump unit.
Also advantageously, the compensation device further comprises check valves connecting each of the chambers of the actuator to the pressurized tank to enable hydraulic fluid to be transferred from the pressurized tank to the chamber of the actuator concerned. The compensation device further comprises pressure-relief valves connecting the pressurized tank to each of the chambers of the actuator to enable hydraulic fluid to be transferred from the chamber concerned to the pressurized tank.
Preferably, the pressurized tank is connected to the pressure-generator device via a constriction to enable the pressurized tank to be filled. It is also advantageously connected to the main supply via a pressure-relief valve.
In an aspect of the invention, a bleed valve is arranged to enable the pressurized tank to be emptied into the main supply so as to enable the fluid in said tank to be renewed periodically.
Finally, the pressurized supply is preferably fitted with a pressure sensor.
Other characteristics and advantages of the invention appear more clearly in the light of the following description of a particular, non-limiting embodiment of the invention.